Waveguide co2 laser with multiply folded resonator

ABSTRACT

A gas-discharge waveguide CO 2  laser has a Z-shaped folded waveguide formed by three ceramic tubes. Ends of the adjacent tubes are shaped and fitted together to form a common aperture. The tubes are held fitted together by spaced-apart parallel discharge electrodes. Four minors are arranged to form a laser-resonator having a longitudinal axis extending through the tubes.

CROSS REFERENCE

This application is a continuation of U.S. application Ser. No.13/100,198, filed May 3, 2011, the disclosure of which is incorporatedherein by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to waveguide CO₂ gas-dischargelasers. The invention relates in particular to CO₂ lasers with multiplyfolded resonators.

DISCUSSION OF BACKGROUND ART

Waveguide CO₂ gas-discharge lasers with output power of about 100 Watts(W) or lower are generally preferred for applications such as productmarking, engraving and fine cutting, where high beam-quality isimportant. In such a laser, a lasing mode in a resonator of the laser iscontrolled by confining the lasing mode in mutually perpendicular,transverse axes in a dielectric waveguide. Radio frequency (RF) power,supplied to electrodes on opposite sides of the waveguide in one of thetransverse axes, creates a gas-discharge in a lasing-gas mixture in thewaveguide. The gas-discharge energizes the lasing gas mixture andthereby provides optical gain in the laser-resonator. The lasing-gasmixture typically is a mixture of carbon-dioxide (CO₂) with inert gases,such as nitrogen and helium.

For any given lasing gas mixture, and RF power applied to theelectrodes, the laser-resonator will have a certaingain-per-unit-length. Because of this, power in a beam delivered fromthe laser is directly dependent, inter alia, on the length of thelaser-resonator.

In order to confine a waveguide CO₂ laser within a convenient space or“footprint”, the laser-resonator is typically “folded” one or more timesby a plurality of mirrors. A detailed description of suchfolded-resonator, waveguide CO₂ lasers is provided in U.S. Pat. No.6,192,061, assigned to the assignee of the present invention, and thecomplete disclosure of which is hereby incorporated herein by reference.A principle feature of these waveguide CO₂ lasers is that thecorrespondingly-folded waveguide is machined, by a grinding process,into a block of a ceramic material, such as alumina. This permitsadjacent waveguide branches at an angle to each other to overlap at the“fold”, and merge into a single aperture. One advantage of such machinedwaveguide branches is that the waveguide branches are held permanentlyin exact alignment with each other. A disadvantage of machined waveguidebranches, however, is that the grinding operation is time-consuming andrelatively expensive. By way of example the cost of a Z-shaped threechannel waveguide can be as high as 17% of the cost of a complete laserwith power-supply. Accordingly, there is a need for a comparable foldedceramic waveguide arrangement that does not require an expensive ceramicmachining or grinding operation.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a gas-discharge laser comprisesa laser housing including a lasing gas. A plurality of ceramic tubes islocated in the laser housing and filled with the lasing gas. Adjacentones of the ceramic tubes are at an acute angle to each other, and withends of the adjacent ones of the ceramic tubes shaped and fittedtogether to provide a common aperture. First and second electrodes arelocated in the laser housing and arranged to create a gas discharge inthe lasing gas in the ceramic tubes when electrical power is applied tothe electrodes. A plurality of mirrors is arranged to form alaser-resonator having a longitudinal axis extending through theplurality of ceramic tubes.

In a preferred embodiment of the invention, the electrodes are spacedapart and parallel to each other with the ceramic tubes locatedtherebetween in corresponding grooves in the electrodes. The ceramictubes are held fitted together by spring force urging the electrodestogether.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, schematically illustrate a preferredembodiment of the present invention, and together with the generaldescription given above and the detailed description of the preferredembodiment given below, serve to explain principles of the presentinvention.

FIG. 1 is a plan-view schematically illustrating a preferred embodimentof a folded-waveguide laser-resonator in accordance with the presentinvention wherein the folded waveguide includes a sub-assembly of threeceramic tubes.

FIG. 1A is an end-elevation view, seen generally in the direction 1A-1Aof FIG. 1, schematically illustrating further detail of the ceramic-tubesub-assembly of FIG. 1.

FIG. 2 is a plan view schematically illustrating detail of individualceramic tubes of the sub-assembly of FIG. 1.

FIG. 3 is an exploded three-dimensional view schematically illustratingan assembly including the ceramic-tube sub-assembly of FIG. 1 andelongated discharge electrodes for exciting gas-discharge in the ceramictubes, the electrodes being configured to hold the ceramic-tubesub-assembly together in the assembly.

FIG. 4 is a fragmentary three-dimensional view schematicallyillustrating detail of the assembly of ceramic tubes and electrodes ofFIG. 3 in one example of a laser-housing.

FIG. 5 is a longitudinal cross-section view seen midway along thelaser-housing of FIG. 4, schematically illustrating further detail ofthe assembly of ceramic tubes and electrodes in the laser-housing.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, wherein like components are designated bylike reference numerals, FIG. 1 and FIG. 1A schematically illustrates apreferred embodiment 10 of a folded waveguide-laser-resonator apparatusin accordance with the present invention. Apparatus 10 includes asub-assembly 12 of ceramic tubes (waveguides) forming in effect a foldedwaveguide. In sub-assembly 12, there are two spaced-apart and parallelceramic tubes 14 having first and second ends 14A and 14B respectively,and a transverse ceramic tube 16 having first and second ends 16A and16B respectively.

End 16A of transverse tube 16 is in contact with (fitted with) end 14Aof one ceramic tube 14, and end 16B of ceramic tube 16 is in contactwith end 14B of the other ceramic tube 14 to impart a “Z”-shape to thesub-assembly with an acute angle θ between parallel ceramic tubes 14 andtransverse ceramic tube 16. The contacted ends of the ceramic tubes arecut such that the tubes combine to form a common aperture 17 as depictedin 1A.

A folded laser-resonator having a longitudinal axis 20 is terminated byend-minors 22 and 24 and folded by fold-minors 26A and 26B. Preferablyone of end minors 22 and 24 is made partially transmissive at afundamental wavelength of the laser-resonator and serves as anoutput-coupling mirror. The other end-minor and the fold-minors aremaximally reflective at the fundamental wavelength.

Where ends of ceramic tubes 14 and 16 are in contact (fitted together)the length of the ceramic tubes are cut back such that common aperture17 has an elongated “waisted” form. This provides that fold minors 26Aand 26B can be spaced apart from the ends of the ceramic tubes whileensuring that that resonator axis 20 is collinear with the longitudinalaxis of the ceramic tubes to optimize lasing efficiency. The minorspacing is required, inter alia, to keep the minors remote from adischarge that will be excited in the ceramic tubes, and to allow foralignment of the mirrors in the resonator.

Extruded alumina capillary tubes suitable for use as ceramic tubes 14and 16 are commercially available from OrTech Inc., of SacramentoCalif.; Sentro Tech Corp., of Berea Ohio; and Coors Tek, of Golden,Colo., among others.

FIG. 2 is a plan view schematically illustrating the ceramic-tubesub-assembly of FIG. 1 in disassembled form. In tubes 14, one end ofeach tube has a length L₁ over which the tube is beveled or chamfered atan angle θ/2 to the longitudinal axis of the ceramic tube. Ceramic tube16 is correspondingly chamfered at to provide the angle θ between theceramic tubes when the chamfered ends thereof are in contact. The lengthL₂ is preferably the same for both ceramic tubes 14 tubes, with ceramictube 16 having an overall length of (L₁+L₂)/Cos θ, such that the ends ofthe ceramic tubes can be aligned in a common plane. In a manufacturingoperation, beveling or chamfering the tubes can be done by batchgrinding, thereby minimizing ceramic machining costs.

The overall length L₁+L₂ of a tube is selected to conform to a desiredlaser length. The number or arms (ceramic tubes) can be only two, ormore than the three exemplified here, in a V-shape, Z-shape, M-shape, orsome more complex shape. Typically about 12.5 W of CO₂ laser outputpower per foot of waveguide length can be obtained. The inside diameter,d, of the ceramic waveguide is selected to satisfy a small FresnelNumber ((d/2)/Lλ) requirement for suppressing high order modes. In a CO₂laser λ (the wavelength of the laser-radiation) will be on the order of10.0 micrometers (μm). A preferred value for the Fresnel number is about0.5.

Angle θ of the waveguide sub-assembly is preferably between about 2° and10°. A larger angle θ increases the width of the waveguide structure andthe width of a corresponding housing for the assembly. A smaller angle θincreases the length L₂ of the overlap of ceramic tubes 14 and 16. Thiscan lead to discharge “hot-spots” within an enlarged discharge region inthe overlap region. Such discharge hot-spots can lead to reduced laserefficiency. Merging ceramic tubes 14 and 16 to form one apertureprovides coupling between discharges in adjacent tubes to ensure thatboth discharges light at about the same time.

A particular challenge in forming ceramic-tube sub-assembly 12 is thatit is not practical to hold ends of ceramic tubes 14 and ceramic tube 16in contact with an adhesive or bonding agent. One reason for this isthat such an adhesive or bonding agent can “outgas” and contaminate thelasing gas mixture. Further, the above-discussed preferred dimensions ofthe tubes and a brittle nature of the ceramic material cause the ceramictubes to be somewhat fragile. It was necessary to devise an assemblymethod that would ensure that the ceramic tubes could be positively heldin contact at the beveled ends thereof; held in correct alignment witheach other such that the longitudinal axes thereof could be maintainedin alignment with the resonator axis; and held in a manner that wouldavoid the ceramic tubes being subjected to mechanic stresses which couldcause flexure or even breakage thereof. A description of one preferredsuch method is set forth below with reference to FIG. 3.

Here an assembly 30, depicted in dissembled form, includes upper andlower electrodes 32 and 34 respectively. These electrodes are forcreating a discharge in the ceramic tubes 14 and 16, when, of course,there is lasing gas in the ceramic tubes and RF-power applied to theelectrodes. In this arrangement it is intended that electrode 32 wouldbe the live or “hot” and electrode 34 the ground electrode. A ceramicplate 48 provides insulation of the hot electrode from a laser housing,which is discussed in detail further herein below.

Electrodes 32 and 34 are furnished with grooves corresponding to theshape and orientation of the ceramic tubes. In electrode 34 parallelgrooves 36 are configured to receive ceramic tubes 14 and an angledgroove 38 is configured to receive ceramic tube 16. There arecorresponding grooves (not visible) in raised portions 40 of electrode32.

In one method of completing assembly 30, the ceramic tubes are placed inthe corresponding grooves in electrode 34; electrode 32 is set withgrooves therein in contact with the ceramic tubes; then the electrodesjoined by inductances 42, with screws 44A engaging threaded holes 46A inelectrode 34, and screws 44B engaging threaded holes 46B in electrode32. Inductances 42 provide for homogenizing a discharge created by theelectrodes, as is known in the art.

The ceramic-tube sub-assembly can be held together in a laser housingwith tubes maintained in alignment by spring pressure applied toelectrode 32, with electrode 34 resting on a base of the housing.Alternatively, inductances 42 can be formed as coil springs, made forexample from phosphor-bronze. These springs can be stretched whilescrewing the springs to the electrodes, and then released to allow thecoil spring tension to urge the electrodes toward each other, and gripthe ceramic-tube assembly. This has an advantage that completion of theelectrode and ceramic tube assembly can be made independent of anoperation of installing the electrodes and ceramic tubes in a housing.

FIG. 4 schematically illustrates one example of above-described ceramictube and electrode assembly 30, located in a laser housing 50. Housing50 is hermetically sealable by an end plate (not shown) via a gasket orO-ring (not shown) in a corresponding groove 51. The end plate wouldtypically include hermetically sealed mounts for the resonator mirrors.A description of one example of such mirror-mounts is provided inabove-referenced U.S. Pat. No. 6,192,061.

In FIG. 4, grooves 41 can be seen in corresponding raised portions ofelectrode 34. The raised portions provide that there is sufficient spacebetween the electrodes, where the electrodes are not in contact with theceramic tubes, to prevent spurious arcing between the electrodes whenRF-power is applied thereto. A preferred height of the raised portionsis between about 0.2 inches and about 0.4 inches.

Further detail of housing 50 is depicted in the longitudinalcross-section view of FIG. 5. Here the cross-section is taken at a pointmidway along the length of housing 50 of FIG. 4, such that a groove 43for accepting ceramic tube 16, in a raised portion 45 of electrode 32 isvisible. The groove and corresponding raised portion extend diagonallyacross electrode 32.

Housing 50 includes two separate compartments 58 and 60. Compartment 58houses the ceramic tube and electrode assembly and the laser resonator,and contains the lasing gas, which of course, is also in the ceramictubes. Web-like or diaphragm portions 51 of the housing enclosingcompartment 58 provide for some flexibility in the housing toaccommodate differential expansion stresses. A detailed description ofthis aspect of the housing is provided in U.S. Patent Publication2009/0213885, assigned to the assignee of the present invention, and thecomplete disclosure of which is hereby incorporated herein by reference.Bulk portions 52 of the housing serve as diffusion-cooling members ofthe housing. In FIG. 5 cooling members 52 are equipped with cooling-fins54. Alternatively, the diffusion-cooling members can be water-cooled.

The sub-assembly of ceramic tubes 14 and 16 can be held together byspring pressure provided by springs 49 compressed between the coolingmember 52 of the housing and ceramic plate 48 which insulateshot-electrode 32 from the housing. Electrode 34 rests firmly on the baseof the housing.

Further, as compartment 58 of the housing contains a lasing gas mixtureat a sub-atmospheric pressure between about 30 to 100 Torr, and becauseof flexibility afforded by diaphragm-portions 51 of the housing,atmospheric pressure surrounding the housing will add to the springpressure provide by springs 49, urging the electrodes together andthereby firmly holding the ceramic-tube and electrode assembly together.Depending on the thickness and flexibility of diaphragm section 51 itmay be possible to rely only on the pressure difference between theinside and outside of the housing to urge the electrodes together.

Compartment 60 houses a RF power-supply (RFPS) 62. The RFPS is mountedon a plate 74, which is furnished with cooling fins 76. A shortconnection 64 connects the RFPS with an electrical feed-through 66. Aconnector 68 of the feed-through transfers the RF power to an aboutmid-way position on the side of electrode 32 facing the connector.

In conclusion, the present invention is described with reference to apreferred embodiment. The invention is not limited, however, to theembodiment described and depicted. Rather the invention is defined byclaims appended hereto.

What is claimed is:
 1. A gas discharge laser, comprising: a laserhousing including a lasing gas; a plurality of independent dielectricwaveguide tubes located in the laser housing and filled with the lasinggas, with adjacent ones of the dielectric tubes thereof at an acuteangle to each other, and with ends of the adjacent ones thereof beingcut to define a taper and fitted together to form a common aperture;first and second electrodes located in the housing and arranged tocreate a gas discharge in the lasing gas in the dielectric tubes whenelectrical power is applied to the electrodes; and a plurality ofmirrors arranged to form a laser-resonator having a longitudinal axisextending through the plurality of dielectric tubes.
 2. The laser ofclaim 1, wherein there are three dielectric tubes fitted together in aZ-shape.
 3. The laser of claim 2, wherein the angle between adjacentones of the dielectric tubes is between about two degrees and about tendegrees.
 4. The laser of claim 1, wherein the electrodes are spacedapart and parallel to each with the dielectric tubes therebetween and incorresponding grooves in the electrodes.
 5. The laser of claim 4,wherein the dielectric tubes are held fitted together by spring forceurging the electrodes toward each other.
 6. The laser of claim 5,wherein the second electrode rests on a base of the housing and thespring force is applied to the upper electrode by one or more springscompressed between the first electrode and the housing.
 7. The laser ofclaim 5, wherein there is a plurality of inductance coils in the form oftensioned springs spaced apart along opposite sides the electrodes, eachthereof connecting the first electrode to the second electrode such thatthe spring tension provides the spring force urging the electrodestoward each other.
 8. The laser of claim 4, wherein the interior of thehousing is at a sub-atmospheric pressure and the housing and electrodesare configured and arranged such that the housing applies pressure tothe electrodes due to atmospheric pressure outside the housing, therebyholding the dielectric tubes fitted together.
 9. A gas discharge laser,comprising: a laser housing including a lasing gas; a plurality ofindependent dielectric waveguide tubes located in the laser housing andfilled with the lasing gas, with adjacent ones of the dielectric tubesthereof at an acute angle to each other, and with ends of the adjacenttubes being cut to define a taper and fitted together to form a commonaperture; first and second electrodes located in the housing andarranged to create a gas discharge in the lasing gas in the dielectrictubes when electrical power is applied to the electrodes, the electrodesbeing further arranged to hold the plurality of dielectric tubes inalignment with each other; and a plurality of mirrors arranged to form alaser-resonator having a longitudinal axis extending through theplurality of dielectric tubes.
 10. The laser of claim 9, wherein thereare three dielectric tubes arranged in a Z-shape.
 11. The laser of claim10, wherein the angle between adjacent ones of the dielectric tubes isbetween about two degrees and about ten degrees.
 12. The laser of claim9, wherein the electrodes are spaced apart and parallel to each with thedielectric tubes therebetween and in corresponding grooves in theelectrodes.
 13. The laser of claim 12, wherein the dielectric tubes areheld fitted together by spring force urging the electrodes toward eachother.
 14. A gas laser comprising: a gas tight housing; a pair of spacedapart, elongated electrodes; a folded waveguide positioned between theelectrodes, said waveguide being formed from at least three independenttubular dielectric waveguide segments, two of said segments beingoriented parallel to each other and parallel to the side edges of theelectrodes and with at least one the said segments being located betweensaid two segments and at non-normal angle thereto and with the ends ofadjacent tubular segments being cut to define a taper and fittedtogether to form a common aperture; and an optical resonator defined bya plurality of minors including at least two end minors, said opticalresonator including at least two fold mirrors being positioned at theopposed ends of said at least one segment for folding the laserradiation emerging from said one segment back into an adjacent segment.15. The laser as recited in claim 14, wherein one of said electrodesincludes a plurality of grooves for seating and aligning said tubularsegments.
 16. The laser as recited in claim 15, wherein the other ofsaid electrodes including a plurality of projections, each projectionincluding a groove for holding an aligning the tubular segments.